Vehicle engine systems may include various vacuum consumption devices that are actuated using vacuum. These may include, for example, a brake booster. Vacuum used by these devices may be provided by a dedicated vacuum pump, such as an electrically-driven or engine-driven vacuum pump. As an alternative to such resource-consuming vacuum pumps, one or more aspirators may be coupled in an engine system to harness engine airflow for generation of vacuum. Aspirators (which may alternatively be referred to as ejectors, venturi pumps, jet pumps, and eductors) are passive devices which provide low-cost vacuum generation when utilized in engine systems. An amount of vacuum generated at an aspirator can be controlled by controlling the motive air flow rate through the aspirator. For example, when incorporated in an engine intake system, aspirators may generate vacuum using energy that would otherwise be lost to throttling, and the generated vacuum may be used in vacuum-powered devices such as brake boosters.
Typically, aspirators are designed to maximize either vacuum generation or suction flow, but not both. Staged aspirators including multiple suction ports or taps may be used, but such aspirators tend to suffer from various disadvantages. For example, staged aspirators may rely on a motive flow of compressed air, and may not be usable in configurations where motive flow is intermittent (e.g., intermittent motive flow may result in vacuum reservoir vacuum loss in some examples). Further, aspirators with multiple suction taps may include one or more taps arranged in the diffuser/discharge cone of the aspirator, e.g., in the diverging portion of the aspirator downstream of the aspirator's throat. A suction tap arranged in the diffuser of an aspirator may act as an initiation site for flow separation, which may render the rest of the diffuser ineffective. Because obtaining a deep vacuum at the throat suction tap of an aspirator is highly dependent on the effectiveness of the diffuser, flow separation caused by any additional suction taps in the diffuser of an aspirator may significantly degrade the aspirator's ability to generate vacuum.
Furthermore, more efficient aspirators may be designed to allow a controlled introduction of suction flow as well as sufficient length for momentum transfer between the motive and suction flows upstream of the diffuser. These features may be difficult to incorporate for suction taps arranged in the diffuser of an aspirator, and thus may often disadvantageously be neglected in staged aspirators.
To address at least some of these issues, the inventors herein have identified a multiple tap aspirator with a design which reduces flow disruption caused by the suction tap in the diffuser and also maximizes forward flow of the diffuser suction tap. In one example, an engine system includes an aspirator, bypassing a compressor, a vacuum source coupled with throat and diffuser taps of the aspirator via respective first and second passages merging into a common passage downstream of the vacuum source, the first and second passages coupled by a leak passage with a flow restriction, a first check valve arranged in the common passage, and a second check valve arranged in the second passage upstream of the leak passage. An exit of the diffuser tap narrows as it approaches the diffuser, and the throat tap and a nozzle of the aspirator together form a converging annular suction flow path into the throat of the aspirator. During conditions when the pressure at the vacuum source is higher than the pressures at the diffuser tap and throat tap, there is forward flow from the vacuum source into both taps (assuming motive flow through the aspirator is present). While the pressure tends to be lower at the throat tap, the flow is restricted there due to the converging annular suction flow path into the throat formed by the throat tap and the nozzle, and thus the majority of the suction flow may advantageously enter the diffuser tap to provide extra suction flow as compared to a single-tap aspirator. Further, in some examples, the exit of the diffuser tap may be substantially parallel to the axis of the diffuser, such that the suction flow entering the diffuser tap is already traveling in the same direction as the motive flow through the diffuser, thereby reducing flow disruption at the diffuser tap.
In contrast, when the pressure at the vacuum source is lower than the pressure at the diffuser tap in the above example, a check valve closes and reverse flow into the diffuser tap (“backflow”) may occur. A special technical effect achieved is that this backflow travels from the diffuser tap into the throat tap, which may advantageously produce an effect similar to bleed-gap or hybrid diffusers where the low-velocity boundary layer is sucked out of the diffuser tap, pulling the high-velocity flow near the wall and decreasing the likelihood of flow separation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.